An image forming apparatus of this type is as that shown in FIG. 59, for example. This image forming apparatus allows to form toner images in a plurality of colors which are different from each other, e.g., four colors of yellow (Y), cyan (C), magenta (M) and black (K), on a photosensitive member 21 which is driven to rotate. The respective toner images are primarily transferred onto a transfer medium 41, such as a transfer belt and a transfer drum, which rotates in synchronization with the photosensitive member 21. The image forming apparatus comprises a drive source 81, such as a dynamotor and a pulse motor, in order to drive the photosensitive member 21 and the transfer medium 41 into rotation. Rotational drive force generated by the drive source 81 is applied to the photosensitive member 21 and the transfer medium 41 through a power transmission unit 9 which comprises power transmission members 91, such as a plurality of gears and a belt, and drives the photosensitive member 21 and the transfer medium 41 into rotation in mutual synchronization.
In this image forming apparatus, toner images in the respective colors are laid one atop the other on the transfer medium 41 through repeated image create/transfer processing for the plurality of colors, and a color image is formed on the transfer medium 41. Following this, the color image is secondarily transferred onto a sheet member S, such as a copy paper, a transfer paper, a paper and a transparent sheet for an overhead projector, which is fed from a cassette or manual-feed tray.
In order to obtain an excellent color image, it is necessary to lay toner images in the plurality of colors one atop the other while registering the toner images with each other. To this end, in the image forming apparatus described above, a sensor 40 for detecting a reference position of the transfer medium 41, for instance, is disposed in the vicinity of the transfer medium 41, and a signal which is outputted from the sensor 40 for every rotation of the transfer medium 41 is used as a reference signal for the image create/transfer processing. More specifically, after a toner image is created on the photosensitive member 21 at predetermined timing for every outputting of the reference signal, the toner images are primarily transferred onto the transfer medium 41 which rotates at a constant speed in synchronization with the photosensitive member 21. As a result, the toner images in the plurality of colors are laid over with each other accurately. Hence, the transfer medium 41 needs be driven to rotate at a constant speed in synchronization with the photosensitive member 21 until the primary transfer completes since outputting of the reference signal from the sensor 40.
However, abutting means 400, such as a secondary transfer roller for secondary transfer onto the transfer medium 41 and a cleaning part for cleaning of the transfer medium 41, sometimes temporarily comes into contact at proper timing, thereby changing loads upon the transfer medium 41, the power transmission members 91, etc. In other words, the contact could hamper the drive rotation of the transfer medium 41, elastically stretch the transfer medium 41, elastically bend the power transmission members 91 in a similar manner, or further, change a load upon a driving part (not shown) which drives the transfer medium 41 into rotation. The contact and separation could prevent the transfer medium 41 from rotating at a constant speed.
In an image forming apparatus of this type, in particular, in order to accurately transmit rotational drive force from the drive source 81 to the photosensitive member 21 and the transfer medium 41, gears formed by a resin material, such as polyacetal (POM), polycarbonate (PC), polyphenylene sulfide (PPS), polybutylene terephthalate (PBT) and polyimide (PI), are often used, and therefore, the gears are elastically deformed as such loads described above change, which is one of main causes of a registration deviation. Further, where the transfer medium 41 is a transfer belt, stretching and shrinking of the transfer medium 41 caused by a change in the loads described above is one of main causes of a registration deviation. A registration deviation which is caused as the abutting means 400 contacts and moves away from the transfer medium 41 will be described in detail in the sections “A-3. Analysis of Causes of Registration Deviation” and “B-3. Analysis of Causes of Registration Deviation” later.
Causes of a registration deviation are not limited to these. A registration deviation may be caused by the following as well. That is, in an image forming apparatus of this type, the photosensitive member 21 and the transfer medium 41 are driven into rotation in mutual synchronization in a sub scanning direction. As the sensor 40 outputs a vertical synchronizing signal using this as a reference, a light beam scans over the photosensitive member 21 in a main scanning direction, which is approximately perpendicular to the sub scanning direction, based on an image signal which is supplied from an external apparatus such as a host computer, whereby an electrostatic latent image which corresponds to the image signal is formed on the photosensitive member 21.
Further, after the electrostatic latent image is developed by a developer with toner and a toner image is formed, the toner image is transferred onto the transfer medium 41 which is driven into rotation in synchronization with the photosensitive member 21 in the sub scanning direction. Such image create/transfer processing is executed for the respective toner colors (yellow, cyan, magenta and black), so that the respective toner images are laid one atop the other and a color image is created on the transfer medium 41.
However, in an image forming apparatus of this type, scan timing of the light beam is not synchronous to the vertical synchronizing signal in many cases, sometimes leading to a synchronization error between the vertical synchronizing signal and the scan timing. In this case, a transfer position on the transfer medium 41 becomes deviated by an amount corresponding to the synchronization error. Since synchronization errors become different among the respective toner colors, the toner images become deviated from each other among the respective toner colors, that is, a registration deviation which degrades an image quality is developed.
The present invention has been made in view of the problem above, and accordingly, aims at providing an image forming apparatus and an image forming method with which it is possible to suppress a registration deviation on a transfer medium and form a high-quality image.